1. Field of the Invention
The present invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine. More particularly, this invention relates to a smooth outer coating for combustor components of a gas turbine component, in which the coating reduces the component temperature by reducing the convective and radiant heat transfer to the component in the combustor section of the turbine.
2. Description of the Related Art
Hot section components of aircraft and industrial (power generation) gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Air plasma spraying (APS) is often preferred over other deposition processes due to relatively low equipment costs and ease of application and masking. TBC's deposited by APS are characterized by a degree of inhomogeneity and porosity that occurs as a result of the deposition process, in which “splats” of molten material are deposited and subsequently solidify. The resulting surface of the TBC is relatively rough, with a surface roughness of 250 to 350 microinches Ra (about 6 to 9 micrometers Ra) being typical for YSZ deposited by APS (APSTBC). The inhomogeneity and porosity of a plasma-sprayed TBC enhances the thermal insulating property of the TBC, and thus helps to reduce the temperature of the component on which the TBC is deposited. In regard to infrared (IR) transmissivity, analysis has shown that APSTBC is about 20% to 70% transparent to thermal radiation (wavelengths of about 780 nm to about 1 mm) when deposited at typically thicknesses of about 250 to 500 micrometers. As a result, the thermal protection provided by APSTBC is compromised in environments that have high thermal radiation loads, such as within the combustor section of a gas turbine.
To be effective, TBC systems must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys though ceramic matrix composite (CMC) materials are also used. To promote adhesion and extend the service life of a TBC system, an oxidation-resistant bond coat is often employed. Bond coats are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during turbine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
The service life of a TBC system is typically limited by a spallation event brought on by thermal fatigue. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can be promoted as a result of the TBC being subjected to substances within the hot gas path of a gas turbine. For example, spallation of TBC from combustor components such as liners, heatshields and transition pieces can be accelerated in industrial gas turbines that burn liquid fuel or utilize water injection for NOx abatement.
In view of the above, further improvements would be desirable for the ability of TBC on combustor components to reject heat and resist spallation.